Studies of several children with hyperphenylalaninemia due to defects in the de novo synthesis of tetrahydrobiopterin (BH4) have shown that the oral administration of either BH4 or a synthetic analogue, 6-methyltetrahydropterin, leads to marked improvement of the neurological symptoms in some, but not all, of these patients. There is a correlation between the ability of the administered pterin to elevate CSF levels of the metabolites of dopamine and serotonin and its ability to improve the neurological status of the patients.